Longitudinal Tendon Layout in Concrete Segmental Bridges

Building a long-span concrete segmental bridge is like conducting an orchestra—every element must work in perfect harmony. But if we had to pick the most critical "instrument" in this symphony, it would be the longitudinal prestressing tendons. These steel strands, carefully threaded through the concrete segments, literally hold the entire structure together. Yet designing their layout is one of the most challenging aspects of segmental bridge engineering.

Having worked on some of major metro projects like the Rach Chiec Bridge in Ho Chi Minh City, Yamuna Bridge in Delhi, Mithi River Bridge in Mumbai and so on.., we've learned that tendon layout design is where engineering theory meets harsh construction reality. Let me share why this seemingly straightforward task is an intricate puzzle that can make or break your project.

Understanding the Real Challenge: Aligning with the Force

The fundamental challenge in tendon layout is aligning the prestressing profile with the bridge's internal force diagram. Ideally, tendons should follow the inverse of the bending moment curve—high at supports to counter negative moments and low at midspan for positive ones. This layout delivers maximum eccentricity and efficient use of prestressing force. But the staged nature of segmental bridge construction disrupts this ideal.

Staged cantilever construction disrupts tendon continuity across the completed spans, demanding multi-phase stress planning.

Unlike cast-in-place bridges, segmental construction proceeds incrementally using cantilever methods. Each segment must carry loads immediately upon placement, making it impossible to design tendons purely for final conditions. Instead, the tendon layout must serve multiple functions: ensuring structural stability during construction as well as before transitioning to final load resistance. For example, in a CLC Bridge like Mithi River Bridge, our team had to fine-tune tendon profile after multiple iterations to finalize the final tendon profile checking for all the intermediate stages as well as service stages.

Navigating Geometry and Physical Constraints

Typical section showing congestion zones limiting tendon placement—especially at webs and anchor blocks.

Even geometry introduces complications. The internal configuration of segmental box girders—defined by thin webs, tight cover constraints, and rebar congestion—often prevents tendons from following their theoretically ideal path. A parabolic drape may be structurally efficient, but if it requires a duct to pass through a 300mm thick web already packed with rebar and transverse tendons, it becomes infeasible. This is where tendon layout becomes an art of compromise—where engineering judgment, construction experience, and creativity must converge.

A Practical Strategy: The Three-Zone Layout Approach

After years of trial and iteration, we’ve found that organizing tendon layout into three functional zones helps strike a balance between ideal force paths and construction feasibility.

Three tendon zones—top slab (green), bottom slab (yellow), and web (blue).

The first zone consists of top tendons located near the top fiber above piers. These resist high negative moments and are typically straight or mildly curved, constrained more by segment geometry and stressing access than by force optimization. On the Rach Chiec Bridge, these tendons were installed just below the track slab to maximize structural height without interfering with overhead systems.

The second zone focuses on midspan tendons, which primarily resist positive moments. These could follow a draped profile—high at supports and dipping low in the bottom slab at midspan. These are the most difficult to construct as their curvature must stay within duct bend radius limits, and their paths must weave through multiple segments and congested webs.

Finally, web tendons form the third zone. These tendons don’t follow a precise theoretical profile but instead act as stress balancers. Effectively positioned at elevations near COG, these cables need to transition from either bottom or top zone to web zone and anchored in Diaphragms.

Field Challenges that Undermine a Perfect Layout

Field photo of stressing area showing need for 3D clearance and access coordination during layout planning.

Even the best theoretical layout can be derailed by practical limitations. One of the most common issues is tendon congestion. Ducts must coexist with transverse prestressing, electrical conduits, and dense rebar, often in highly constrained geometries. Without thorough 3D coordination, conflicts are inevitable—and costly.

Construction access is another recurring obstacle. Anchor blocks must be reachable by jacks, which require 1.5 - 2 meters of working clearance, along with safe platforms and crane access. If stressing points aren’t accessible, tendons simply can’t be activated. And then there’s the issue of tolerance accumulation. Segmental bridges are built from individually cast segments, each with minor dimensional variations. These small discrepancies add up. By the time you reach a 100-meter cantilever tip, ducts can be significantly misaligned. Your layout must be robust enough to absorb these geometric imperfections without causing tendon threading failures.

Coordinating with Pre-Compensation Force Methods

Pre-compensation force application for Yamuna Bridge at closure location.

Tendon layout design becomes even more intricate when integrated with Pre-Compensation Force Methods (PFM). These methods apply temporary hydraulic forces prior to closure to offset future creep and shrinkage effects. But introducing these forces changes the stress state—and therefore affects tendon design.

On the Yamuna Bridge, for instance, some tendons had to be staged—stressed after applying pre-compensation and casting of stitch concrete or closure pour. This required intermediate anchorages, thoughtful access planning, and multi-phase force balancing. If your layout doesn’t accommodate this complexity, both the pre-compensation system and the tendon system can fail to deliver their intended benefits.

Key Lessons from the Field

Designing tendon layouts for segmental bridges teaches you a lot—and often the hard way. First, always start from the construction constraints and work backward. Trying to retrofit constructability into a structurally perfect layout leads to headaches, delays, and redesigns.

Standardization pays huge dividends. While every span may be slightly different, repeating duct geometries across segments minimizes fabrication errors and construction delays. Also, expect things to go wrong. Tendons may snap. Your layout must have enough redundancy and flexibility to absorb these setbacks without compromising the structure.

And most importantly, think in four dimensions. Your tendon layout must not only work in space, but across time—supporting the bridge through all its construction stages and into long-term service.

Harnessing Automation: TendonWorks for Intelligent Tendon Design

The future of tendon layout design isn’t just about automation—it’s about intelligent automation. At the heart of this evolution is TendonWorks, a specialized tool that automates the tendon design and documentation process by removing manual bottlenecks, enabling rapid iterations, and design checks.

Conventional Cross-section showing a typical Section with Tendons using manual Workflow.

Historically, tendon layout documentation was one of the most laborious aspects of segmental bridge engineering. For every tendon, engineers had to extract coordinates at 2–3 meter intervals, often across 30 to 50 tendons, and then format this data for structural analysis.

On complex bridges like Yamuna Bridge, this meant generating over 100 section views, manually extracting thousands of data points, and cross-referencing these across drawings and spreadsheets. Each design revision restarted the cycle. The process could take 6–8 weeks, consuming entire engineering teams and risking human error at every step.

Automated Cross-Section and table generation from TendonWorks.

TendonWorks changes that. With its ability to automatically generate section cuts along curved alignments, compute 3D tendon coordinates with millimetric precision, and export directly to MS Excel and structural software, it condensed an eight-week process into just one week.

On the Mithi River Bridge, the documentation time was cut by over 90%. More importantly, the application's ability to regenerate updated tendon data in minutes gave us freedom to iterate—something previously avoided due to time constraints. This created the possibility of reconfiguring the tendon profile entirely. Initial manual layouts caused significant plan-twist and transverse deviation due to the bridge’s tight curvature and asymmetric stiffness. By using TendonWorks to simulate multiple layouts and compare their induced forces and displacements, we arrived at a tendon configuration effectively reducing the prestressing quantity by over 10%.

Final Thoughts: Science, Art, and Software in Balance

Longitudinal tendon layout in segmental bridges will always blend structural science, construction art, and now, software intelligence. The ideal profile is only ideal if it can be built, stressed, and maintained. The ability to visualize and optimize layout across space and time—with precise documentation—has become the modern standard.

Technology doesn’t just speed things up—it enables better engineering. When documentation bottlenecks are removed, we explore more options, refine our designs further, and deliver better bridges. The elegance of a segmental bridge rests not just on concrete and steel, but on the hidden web of tendons inside—each placed with careful planning, adaptive logic, and increasingly, the help of intelligent tools.

Completed segmental transit viaduct—structural elegance enabled by hidden tendon precision.